1. Field of the Invention
This invention relates to color projection systems, more particularly to color projection systems that use a white light source.
2. Background of the Invention
The use of white light sources in color projection systems offer many advantages. They are relatively inexpensive, use low amounts of power, and are easier to install and replace. Other options, such as lasers, or Light-Emitting Diodes (LEDs), are expensive, use large amounts of power and require three sources, one for red, green, and blue, respectively.
White light also offers the full color spectrum, which can be filtered to select a desired color, such as red, green and blue. This becomes important when the light is to be directed to the projection surface, as is done when spatial light modulators are used. Normally, the spatial light modulator is positioned between the source or sources, and the projection surface, be it a television, a printer drum, a piece of photographic film, or any photosensitive media. The light from the source impinges upon the modulator that, in turn, selectively directs the light to the projection surface. In this type of system, the color of the light is normally done with filters.
The color filtering can be done either spatially or temporally. In order to spatially color filter, three sets of arrays, or three sections, such as rows on one array, could be filtered for one of the three required colors. The key in this instance is to separate the colors and direct one color onto its own dedicated space, such as a separate modulator device, or an area upon one device.
The other option is to temporally filter the light. One way is to use a rotating filter that is divided into equal fields of the three colors. This is normally called a color wheel. The standard wheel is divided into three arcs of the circle, each being 120.degree.. The wheel then spins in front of the source, relying the appropriate amount of each color to the projection surface. The amount of each color is dependent on how long the cells of the modulator pass the colored light to the surface.
An inherent trade-off exists in any color projection system. If the desire is for color saturation, there will be a drop-off in overall brightness. Likewise, if overall brightness is desired, some loss in color saturation will possibly result. In many cases, viewers have been found to favor brightness over color saturation. A problem exists in standard color projection techniques, which tend to favor color saturation over brightness. Therefore, a way is needed to adapt the current methods of color projection to provide better brightness, without having too great an effect upon color saturation.
The initial solution to this problem would be to simply use a brighter light. Depending on the system limitations and requirements, it may not be possible to use a bright enough source to alleviate this problem. Therefore, some alternate solution must be found.